Structural ceramics typically have many properties, e.g., high strength at elevated temperatures, excellent resistance to chemical degradation and wear, and low density, that make them attractive materials for many high performance applications. High fabrication costs and uncertain reliability, however, create considerable barriers to the utilization of these materials. Both the cost and performance of ceramic materials are strongly influenced by machining operations. Machining costs make up a large component of the cost of structural ceramics, sometimes constituting as much as 90% of the total fabrication costs. In addition, the performance and reliability of ceramic articles are strongly influenced by the presence of machining-induced damage.
Ceramic materials are generally more difficult to machine than metals. Ceramic materials are defined as non-metallic inorganic materials which are formed via high temperature processing. Because of their hardness, the machining of ceramic materials usually requires a normal (feed) cutting force that is higher than that required for metals. Additionally, while metals are ductile, ceramic materials are typically quite brittle. As a result of these factors, the machining of ceramic materials is likely to produce machining-induced damage in the finished ceramic article and lead to increased cutting tool wear rates (in comparison to metals). Overall, the machining of ceramic materials places stringent demands on the performance of cutting tools and cutting fluids. The latter are customarily used to provide cooling and lubrication during machining operations.
Where possible, it is preferable to carry out the machining of ceramic materials through conventional cutting operations, such as turning, milling or drilling. These operations are capable of producing articles with high precision. For example, turning of ceramic materials using a single diamond cutting point is typically employed to produce highly smooth and precise surface contours for specialized applications, such as optical components. However, with cutting operations such as turning, special precautions to minimize tool wear are required. Damage to the cutting point may lead to a failure to achieve desired tolerances and can contribute to an increase in the machining-induced damage in the finished ceramic article.
Many ceramic materials are too hard to be machined by cutting techniques and may only be fabricated by abrasive machining operations, such as grinding and polishing, which are characterized by low productivity. As a result, the use of such ceramics is often not favored in comparison to metal superalloys, which may be fabricated more readily. In order for ceramic materials to compete directly with metals in many high performance applications, significant advances in machining techniques to permit the rapid, economical fabrication of ceramic articles must occur.
Most nitride and carbide ceramics are too hard to be machined using cutting techniques. Oxide ceramic materials, which typically are somewhat easier to machine, offer more promise as potential workpieces for these rapid machining methods. An oxide ceramic is defined to be any ceramic material, which includes a substantial amount, i.e., at least about 20%, of an inorganic oxide (e.g., alumina, silica, aluminosilicate or zirconia). There is considerably more experience with the design and fabrication of oxide ceramics than with other ceramic materials. In addition, for many applications oxide ceramic materials may possess better resistance to chemical degradation than metal alloys or other ceramics. Further, oxide ceramics are, as a rule, substantially less expensive than nitride or carbide ceramics. In view of these advantages, oxide ceramics appear to offer the most potential to satisfy the demand for readily and economically fabricated ceramic materials.
In comparison to abrasive methods of machining ceramics, such as grinding or polishing, cutting operations typically generate higher levels of machining-induced damage in a finished ceramic workpiece. In order to permit ceramic articles to be routinely fabricated, cutting methods which avoid excessive machining-induced damage to the workpiece and achieve acceptable tool wear rates must be available.
In cutting a ceramic workpiece, the removal of material occurs in the cutting zone, i.e., the interface between the cutting point (or points) and the workpiece surface. In this interaction, the workpiece surface undergoes elastic and plastic deformation, followed by the fracture of small particles or chips from the surface. Whether deformation or fracture dominates the removal process depends on the properties of the workpiece material and the cutting conditions. With ceramics that exhibit a low toughness, the removal of material often occurs by a brittle fracture process, resulting in a machined surface that contains damage in the form of microcracks. Since the processes of deformation and fracture are related to the forces applied at the interface of the workpiece surface with the cutting point, any reduction of these forces decreases the tendency of the workpiece to fracture in an uncontrolled manner. In the cutting of a ceramic workpiece, cutting conditions that increase the removal rate, while at the same time minimizing the level of machining-induced damage, are to be desired.
Cutting fluids may have a substantial effect on cutting efficiency and tool wear, as well as on the surface finish and the surface and subsurface damage of the finished ceramic article. In addition to reducing contact forces, which may be accomplished by using additives in the cutting fluid that reduce the coefficient of friction, the cutting process may also be improved by controlling the temperature during the cutting operation. The ability of a cutting fluid to remove heat is an extremely important factor, since the thermal stresses associated with high local temperatures may lead to the formation of large microcracks during the cutting of ceramic workpieces with low thermal conductivity. These microcracks may later lead to the fracture and failure of the finished ceramic article. The reduction of friction at the cutting zone decreases the overall temperature in cutting, since approximately 50% of the heat is generated from sliding in the cutting zone, i.e., at the tool/workpiece and the chip/tool interfaces. The remaining heat is generated from deformations of the workpiece in the shear zone.
In general, cutting fluids used in machining may be classified into three groups: mineral oils, soluble oils and chemical (synthetic) fluids. Of these three, both soluble oils and chemical fluids are water-based. Minerals oils, which include a variety of performance enhancing additives, are generally used in the low speed grinding of ceramics and in metal cutting operations. Mineral oil cutting fluids typically have very good lubricating properties but do not perform as well as water-based cutting fluids in controlling temperature.
In cutting operations with a large degree of heat generation, as for example in the cutting of a ceramic workpiece, water-based cutting fluids are typically employed due to the high heat capacity of water. Conventional soluble oils and chemical fluids, however, both suffer from disadvantages.
Soluble oils are emulsions of oil in water, generally containing a much greater amount of water than oil. While soluble oils may have the high heat capacity of water-based fluids in addition to the lubricating properties of mineral oils, a major drawback of soluble oil cutting fluids is their milky color. This milky color may obscure vision in the cutting zone.
Chemical fluids are aqueous solutions of water-soluble additives, which usually are present as about 5-10 wt. % of the cutting fluid. While these fluids have excellent cooling capacities and are transparent, chemical fluids do not typically have the lubricating capability of either mineral oils or soluble oils.
As enumerated above, conventional cutting fluids have a number of drawbacks for machining ceramic workpieces and, in particular, for use in cutting ceramic workpieces where higher temperatures may be experienced. The environmental aspects and disposal problems associated with expended cutting fluids are also extremely important issues in the design and selection of cutting fluid additives. Additives should, if at all possible, preferably be non-toxic, non-flammable, biodegradable, and not present any handling problems. There is, accordingly, a continuing need for safe, inexpensive water-based cutting fluids with effective friction reducing capabilities, which may be used in the cutting of ceramic workpieces, and in particular, which may be used in cutting oxide ceramic materials.
Several publications have shown that a coating of solid boric oxide or solid boric acid (i.e., essentially free of any liquid) may act as a lubricant in reducing the coefficient of friction between contacting surfaces. Typically, the solid coating is formed on at least one of the surfaces to be lubricated using either boric acid or boric oxide in powdered form.
Boric acid has also been proposed and/or employed as an additive in metal cutting fluids, which may be used in the cutting, grinding, polishing, or forming of metals. In many of these applications, the boric acid is reacted with amines, fatty acids, alcohols, or other hydrocarbons to form chemical adducts that are utilized for friction reduction, corrosion protection, and as bactericides and fungicides. In other applications, boric acid and hydrocarbon compounds are mixed with water for use as cutting fluids. In such cases, although the boric acid is not intentionally reacted with the hydrocarbon compounds, chemical adducts are believed to be formed under the cutting conditions.
In addition, boric acid has been reported as being among a number of additives in multi-component mixtures used in conjunction with the drilling, polishing or grinding of rocks, refractories, glass or ceramic articles. For example, aqueous solutions, which include sodium tripolyphosphate, sodium tetraborate, triethanolamine and boric acid together with other additives such as hydrofluoric acid, ammonium flourosilicate, or hexamethylenetetramine, have been employed as cutting fluids for the processing or machining of ceramic articles. Multicomponent mixtures such as these are more likely to be expensive, may prove to be more difficult to optimize and are more likely to present environmental and/or handling issues.